Seawater and brackish water desalination technologies hold great promise to alleviate water scarcity in arid and densely populated regions of the world. Increasing population growth and a warming global climate have created ever greater disparities between the supplies of, and demands for, reliable fresh water sources. In several cases, conflicts over shared water resources have exacerbated already significant tensions between neighboring states.
Even in areas with sufficient water supplies, inconsistent and often poor water quality contributes to disease and suffering that would be much less prevalent were adequate water treatment more widely available. The need to alleviate water scarcity and ensure good water quality will be a major challenge for scientists and engineers in the coming century. Much work has been done to improve existing water treatment technologies, particularly with regard to increasing the effectiveness and lowering the cost of membrane treatment methods. In a membrane treatment method, a semi-permeable membrane, like the cell wall of a bladder, is used that is selective about what it allows through, generally allowing small molecules (such as water) to pass easily but preventing the passage of many other compounds. With the presence of two solutions, each containing a different concentration of dissolved compounds on either side of the barrier, water will typically move from the side of the more dilute solution to the more concentrated solution. Eventually, osmotic pressure will counter the diffusion process exactly, and equilibrium will form.
One membrane treatment method is known as Reverse Osmosis (RO) which is well understood by one of skill in the art of desalination. The process of RO forces a net flow of water molecules from an aqueous solution with a greater concentration of compounds present within it through a semi-permeable membrane and into a solution with a lower concentration of dissolved compounds. High water pressure on the source side is used to “reverse” the natural or forward osmotic process.
Although progress has been made in lowering the energy requirements and, thus, the cost of RO, challenges remain to be overcome. The energy costs of seawater and brackish water RO are still too high for economic widespread use; large brine discharge streams continue to cause concern over the environmental impacts they may cause; and long term equipment replacement costs remain significant.
In an effort to address some of the challenges still facing current seawater and brackish water desalination technologies, methods of ammonia-carbon dioxide forward osmosis (FO) desalination have been developed. FO processes are described, for example, in U.S. Pat. No. 6,391,205 and U.S. Patent Application Publication No. 2005/0145568, now U.S. Pat. No. 7,560,029, the contents of which are incorporated herein by reference in their entirety. Key advantages of the FO process as compared to RO include lower energy costs, high feedwater recovery, and brine discharge minimization.
In the ammonia-carbon dioxide FO process, a semi-permeable membrane of a type similar to that used in RO is used to separate fresh water from a saline feedwater source. In RO, this separation is driven by a hydraulic pressure gradient across the membrane, generated to a magnitude significantly in excess of the osmotic pressure gradient which resists the flow of fresh water (permeate flow) from the saline feedwater source. The FO process uses the natural tendency of water to flow in the direction of higher osmotic pressure (towards a more concentrated solution), to draw water from the saline feed stream into a highly concentrated “draw solution”, effectively separating the fresh water permeate from the saline feedwater stream. A schematic diagram of a prior art ammonia-carbon dioxide FO process is shown in FIG. 1.
The membranes used in the ammonia-carbon dioxide FO process are similar to those used in the RO process. One significant difference lies in the high hydraulic pressures that RO membranes must sustain. This requirement leads to the use of a supporting fabric layer (often up to 100 microns in thickness) within the membrane to increase its strength, an addition which significantly diminishes flux performance when membranes of this type are used in an FO process. FO tests conducted using a membrane specifically manufactured for FO, such that no fabric backing layer was included in its design, demonstrated a flux performance over ten times higher than fabric-backed RO membranes of similar chemistry.
The negative impact on FO performance associated with RO membranes is due to internal concentration polarization (ICP) of the draw solution within the membrane fabric support. In this phenomenon, the permeate penetrating the dense membrane (rejecting layer) dilutes the draw solution within the supporting layer, such that the effective osmotic pressure is greatly diminished at the dense membrane surface. The rate of solute diffusion in the direction of the dense layer is in most cases insufficient to completely counteract the dilution caused by the water flux away from it. This phenomenon may not be counteracted by increasing the tangential flow rate or turbulence of the draw solution, steps normally effective in reducing external concentration polarization, as the ICP phenomenon takes place within the confines of the porous support.
While the elimination of the fabric layer from the membrane's construction improves FO flux significantly, the effects of ICP are not completely resolved by this modification. Some effects remain within the non-fabric porous polymer support integral to asymmetric or thin film composite membrane structures. This porous layer, approximately 50 microns in thickness, underlies the dense membrane layer where solute rejection occurs. The dense layer, often only several microns thick, must be reinforced by this supporting structure in order to sustain handling and fluid shear forces which would otherwise tear the membrane surface. This results in continued reduction in effective osmotic pressure relative to that which would be realized if a dense separating membrane were used alone.
The reduction in effective osmotic pressure due to ICP may be expressed in terms of a “membrane performance ratio” (Pm), defined as the ratio of experimental, or measured flux (Jexp), to theoretical flux calculated from the osmotic pressure difference between the feed and draw solutions (Jthr):
      P    m    =            J      exp              J      thr      
The membrane performance ratio in FO can be quite low, in some cases as little as 2-3%, even when using a membrane designed specifically for FO. The inefficiencies of low membrane performance ratios are not limiting to FO process operation, however, so long as sufficiently high draw solution concentrations are employed. It has been demonstrated that membrane flux could be established equivalent to, or in excess of, that typical of RO, and that seawater recoveries of up to 75% were achievable, based on effective separation of water from a 2 molar NaCl feed stream.
For effective FO desalination, the draw solution must have a high osmotic pressure and contain solutes which are simple and economic to remove and reuse. In the ammonia-carbon dioxide FO process, the draw solution is composed of ammonium salts formed from the mixture of ammonia and carbon dioxide gases in an aqueous solution. The salt species formed include ammonium bicarbonate, ammonium carbonate, and ammonium carbamate. Of these, ammonium carbamate is by far the most soluble. Other draw solutions may utilize ethanol and other thermally removable draw solutes.
One important characteristic of ammonia-carbon dioxide draw solutions is the ratio of ammonia to carbon dioxide in the ammonium salts. The higher the ratio of ammonia to carbon dioxide in the draw solution, the higher the concentration of ammonium carbamate relative to other dissolved species. This allows for a higher concentration of total ammonium salts, leading to a higher osmotic pressure within the solution. The maximum solubility of ammonium bicarbonate at room temperature, for instance, is about 2 molar, but addition of ammonia to such a solution favors the formation of ammonium carbamate (and to a much lesser extent, ammonium carbonate), which allows further carbon dioxide to be added, and so on, allowing high total concentrations of ammonium salts to be dissolved. Elevation in solution temperature also leads to some elevation in solute solubility, but the primary mechanism responsible for high draw solution concentrations is the ratio of the gases that form the salts. The generation of high osmotic pressures in turn allows for the generation of both high water fluxes and high feedwater recoveries in the FO desalination process.
Once the osmotic pressure gradient created by the FO process causes fresh water to flow across the membrane from the saline water feed into the draw solution, the diluted draw solution must be treated for the separation of the ammonium salts. This separation process (also referred to as the recovery process) is based on the thermal decomposition of ammonium bicarbonate, carbonate and carbamate salts into ammonia and carbon dioxide gases that occurs when a solution containing these solutes is heated at an appropriate temperature and pressure. At atmospheric pressure, this decomposition occurs at about 60° C. At lower pressures, the decomposition temperature decreases proportionally. This heating, decomposition, and the stripping and recycling of the ammonia and carbon dioxide gases may be accomplished in a single or in multiple distillation columns, producing as its products fresh water and re-concentrated draw solution for reuse in the FO membrane system. The product water from this process may be specified to contain significantly less than 1 ppm ammonia and carbon dioxide, as is appropriate for potable use.
A simple and proven approach to the removal and recycling of draw solutes from the dilute FO draw solution is the use of a distillation column, which is also known as a reboiler absorption column, or stripper. This approach is now employed, for example, in the stripping of various volatile solutes from wastewaters and process streams, and for the recycling of ammonium carbamate as ammonia and carbon dioxide gases in the production of urea. Depending on the temperature of the heat to be used in the recovery system, one or several distillation columns may be used.
A simple and low energy cost approach to solute recovery in the FO process is the use of a single vacuum distillation column. A schematic of a typical prior art single vacuum distillation column is shown in FIG. 2. This configuration is especially useful when the source of thermal energy is at low temperatures, from about 40 to about 44° C. As shown in FIG. 2, heat at temperatures as low as 40° C. is introduced to a heat transfer means, here, the exterior of the heat exchange surface of reboiler (1) to induce water vapor to rise in a distillation column (a) as the dilute draw solution (introduced at the top of the column) (2) cascades downward in counter-current flow. The transfer of energy from the rising vapor to the falling liquid causes fractional separation of the more volatile ammonia and carbon dioxide from the less volatile water, such that higher in the column there is a higher fraction of ammonia and carbon dioxide than at points lower in the column. At steady state operation, the product water (3) exiting the bottom of the column may be specified to contain less than 1 ppm of ammonia and carbon dioxide. The recovered/separated solutes are introduced back into the concentrated draw solution through an outlet (4) of the distillation column (a). The energy required for this approach is almost entirely thermal, with a small amount of additional electrical power used for fluid pumping to and from the column.
As noted previously, key advantages of the FO process include lower energy costs, high feedwater recovery, and brine discharge minimization. The inventors of the present invention have determined that the cost for the thermal energy required in a FO desalination process may be lowered even further if the efficiency of heat use is improved. The inventors have further determined that this may be accomplished by using higher temperature heat sources in conjunction with a plurality of distillation columns. This approach improves the efficiency of heat use and cuts the energy use of the recovery process (separation of draw solution solutes and product water from the draw solution) by over 70% relative to the use of a single distillation column.